I was confused after the announcement of the  Watch - partially because it wasn't what I was expecting, and partially because the presentation was muddled and generally not up to Apple's standards. They never really answered the question 'why does this exist?'. After thinking about it some more and reading Ben Thompson’s piece, I’m convinced that Apple’s ambition is to create a general purpose computing platform (eventually with its own cellular and wifi connections, GPS, and the ability to run native apps, all without an iPhone). That's what Apple does: they design great hardware with the user experience in mind, make some nice apps for it, and let developers create new and interesting experiences for the platform. They've done this with the Mac, iPhone, and iPad. The iPod is their only major product category that hasn't fit this description.
The iPhone and other smartphones that followed made several product categories all but obsolete: mp3 players, point and shoot cameras, portable GPS navigation units, PDAs, and more. I was thinking about the Apple Watch the other day on my run. I always wear a watch when I run to keep track of my time. Sometimes I take my phone to use a GPS running app and listen to music. If I don't have my phone I usually take an iPod Shuffle. All of those products would be unnecessary if I had an Apple Watch (well, a future version that had its own GPS). I think future versions of the Apple Watch could kill or take a big bite out of: pedometers and activity trackers, GPS watches and heart rate monitors, and small mp3 players such as the iPod Shuffle.

These categories are smaller than the categories smartphones killed. In fact, they probably don't add up to a market large enough to support the Apple Watch's R&D budget and ambitions. The smartphone market grew to be massive and is much larger than the combination of all the products it replaced. Apple is clearly betting that its watches can do something similar, perhaps on a slightly smaller scale. However, smartphones also made one other product category obsolete: feature phones, which already had a huge and lucrative market of their own. Furthermore, cellular phones had already made wristwatches unnecessary for many people, or relegated them to specific activities and occasions.

Wristwatches are the category smartwatches could most clearly make obsolete. There is still a large market for wristwatches, but it is very fragmented with price points from $5 - $50,000 and up, whereas feature phone prices were all on the same order of magnitude with a few exceptions. Many people wear watches as fashion pieces and expressions of their individual personality as much as they do for the utility they provide. It's not clear to me that the Apple Watch can make a significant number of these timepieces obsolete the way smartphones made feature phones obsolete.

The question I have for the (future version of the) Apple Watch, then, is will it do enough to convince lots of normal people to buy it. People that wouldn't otherwise buy a Fitbit or Pebble smart watch or $400 GPS running watch. People that don't wear a watch every day. I'm confident a few years from now the hardware will be at a point where the watch will work without a smartphone with a day or more of batter life. Will developers create enticing use cases that only work on a watch (just like Uber and Snapchat wouldn't have worked without smartphones)? I can also imagine the  Watch being a fashion item and status symbol. I just don't know if all of that adds up to a large enough market. I'm not betting against Apple, though.